Systems and methods for selectively producing steam from solar collectors and heaters for processes including enhanced oil recovery

ABSTRACT

Systems and methods for selectively producing steam from solar collectors and heaters, for processes including enhanced oil recovery, are disclosed herein. A system in accordance with a particular embodiment includes a water source, a solar collector that includes a collector inlet, a collector outlet, and a plurality of solar concentrators positioned to heat water passing from the collector inlet to the collector outlet, a fuel-fired heater, a steam outlet connected to an oil field injection well, and a water flow network coupled among the water source, the solar collector, the heater, and the steam outlet. The system can further include a controller operatively coupled to the water flow network and programmed with instructions that, when executed, direct at least one portion of the flow through the solar collector and the fuel-fired heater in a first sequence, and direct the at least one portion or a different portion of the flow through the solar collector and the fuel-fired heater in a second sequence different than the first sequence.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 61/749,888, filed Jan. 7, 2013, and U.S. Provisional Application No.61/829,984, filed May 31, 2013, both of which are incorporated herein byreference. To the extent the foregoing applications and/or any othermaterials incorporated herein by reference conflict with the presentdisclosure, the present disclosure controls.

TECHNICAL FIELD

The present technology is directed generally to systems and methods forselectively producing steam from solar collectors and heaters, includingtechniques for increasing the steam production efficiency for enhancedoil recovery processes by varying the energy contributions from each ofthe foregoing components.

BACKGROUND

As fossil fuels become more scarce, the energy industry has developedmore sophisticated techniques for extracting fuels that were previouslytoo difficult or expensive to extract. One such technique is to injectsteam into an oil-bearing formation to free up and reduce the viscosityof the oil. Several techniques for steam injection presently exist, andare often collectively referred to as “Thermal Enhanced Oil Recovery,”or “Thermal EOR.” Representative steam injection techniques includecyclic, steamflood, steam-assisted gravity drainage (SAGD), and otherstrategies using vertical and/or horizontal injection wells, or acombination of such wells, along with continuous, variable-rate, and/orintermittent steam injection in each well.

One representative system for forming steam for steam injection is afuel-fired boiler, having a once-through configuration or arecirculating configuration. Other steam generating systems include heatrecovery steam generators, operating in a continuous mode. Thermal EORoperations often produce steam 24 hours per day, over a period rangingfrom many days to many years. During the period of continuous steaminjection, steam pressures and flow rates may be roughly constant insome cases, may vary within a predetermined limited range in othercases, or may vary more widely according to a predetermined plan.However, it is generally undesirable to shut down and start the steamgenerating equipment (e.g., on a daily basis) because such cyclesincrease equipment maintenance associated with thermal cycling, and thepotential for increased corrosion during idle or standby periods.

Another representative steam generator is a solar steam generator, whichcan augment or replace the fuel-fired boilers. Solar steam generatorscan reduce fuel use, reduce operations costs, reduce air emissions,and/or increase oil production in thermal recovery projects. However,such systems collect energy intermittently due to day/night patterns ofavailable solar radiation, with energy production rates varyingaccording to factors which include the variation of incoming solarradiation. Due to the above-described demand for continuous steam, manyor most thermal recovery projects which incorporate solar steamgenerators will also incorporate one or more fuel-fired steamgenerators, which combust liquid or gaseous fuels as a supplementalsource of heat to form steam from an incoming liquid water stream.However, such combinations of solar and fuel-fired boilers are typicallyinefficient, e.g., because they include many additional components,waste energy, and/or have excess or redundant capacity. Accordingly,there remains a need for high-efficiency solar steam generation systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic illustration of a system that includes asolar collector, a heater, and a controller configured in accordancewith an embodiment of the presently disclosed technology.

FIG. 2A is a partially schematic illustration of the system shown inFIG. 1, configured to operate at night in accordance with an embodimentof the presently disclosed technology.

FIG. 2B is a partially schematic illustration of the system shown inFIG. 1, configured to operate under conditions of relatively lowincident radiation, in accordance with an embodiment of the presentlydisclosed technology.

FIG. 2C is a partially schematic illustration of the system shown inFIG. 1, configured to operate under conditions of relatively highincident radiation, in accordance with an embodiment of the presentlydisclosed technology.

FIG. 2D is a partially schematic illustration of the system shown inFIG. 1, configured to operate under peak incident radiation conditions,in accordance with an embodiment of the presently disclosed technology.

FIG. 3 is a partially schematic illustration of a system configured inaccordance with another embodiment of the present technology.

FIG. 4 is a partially schematic illustration of a system configured inaccordance with still another embodiment of the present technology.

FIG. 5 is a graph illustrating representative heat contributions from asolar collector and heater in accordance with several embodiments of thepresent technology.

FIG. 6 is a graph illustrating representative variations in solarradiation levels over the course of a day.

FIG. 7A is a partially schematic illustration of a system that includesa storage medium configured in accordance with an embodiment of thepresent technology.

FIG. 7B is a partially schematic illustration of an oil field servicedby multiple combinations of solar collectors and heaters in accordancewith an embodiment of the present technology.

FIG. 7C is a partially schematic illustration of a system having astorage medium and separator configured in accordance with anotherembodiment of the present technology.

FIG. 7D is a partially schematic illustration of a system having avalving arrangement configured in accordance with still anotherembodiment of the present technology.

FIG. 7E is a partially schematic illustration of a system having avalving arrangement in accordance with yet another embodiment of thepresent technology.

FIGS. 8A-8C are partially schematic illustrations of systems havingpiping and valve configurations in accordance with still furtherembodiments of the present technology.

DETAILED DESCRIPTION

The present technology is directed generally to systems and methods forselectively producing steam from solar collectors and one or moreadditional heaters (e.g., auxiliary, complementary and/or supplementaryheaters). Specific details of several embodiments of the disclosedtechnology are described below with reference to a system configured foroil well steam injection to provide a thorough understanding of theseembodiments, but in other embodiments, representative systems can beused in other contexts. Several details describing structures orprocesses that are well-known and often associated with steam generationsystems, but that may unnecessarily obscure some significant aspects ofthe present technology, are not set forth in the following descriptionfor purposes of clarity. Moreover, although the following disclosuresets forth several embodiments of different aspects of the presentlydisclosed technology, several other embodiments of the technology canhave configurations and/or components different than those described inthis section. Accordingly, the presently disclosed technology may haveother embodiments with additional elements, and/or without several ofthe elements described below with reference to FIGS. 1-8C.

Many embodiments of the presently disclosed technology described belowmay take the form of computer-executable instructions, includingroutines executed by a programmable computer. Those skilled in therelevant art will appreciate that the technology can be practiced on orby computer systems and/or controllers other than those shown anddescribed below. The technology can be embodied in a special-purposecomputer or data processor that is specifically programmed, configuredand/or constructed to perform one or more of these computer-executableinstructions described below. Accordingly, the terms “computer” and“controller” as generally used herein refer to any suitable dataprocessor and can include internet appliances and hand-held devices(including palm-top computers, wearable computers, cellular or mobilephones, multi-processor systems, processor-based or programmableconsumer electronics, network computers, minicomputers and the like).

The present technology can also be practiced in distributedenvironments, where tasks or modules are performed by remote processingdevices that are linked through a communications network. In adistributed computing environment, program modules or subroutines may belocated in local and remote memory storage devices. Aspects of thetechnology described below may be stored or distributed oncomputer-readable media, including magnetic or optically readable orremovable computer disks, as well as distributed electronically overnetworks. In particular embodiments, data structures and transmissionsof data particular to aspects of the technology are also encompassedwithin the scope of the present technology.

Embodiments of the technology described below include systems thatgenerate steam, techniques for manufacturing, assembling, programming,operating and/or controlling such systems, and methods for instructingthe use, construction, development, and/or other aspects of the system.Accordingly, various embodiments of the presently disclosed technologymay include taking direct action (e.g., by a person, machine,controller, software or hardware) and/or instructing (by a person,machine, controller, software or hardware) tasks to be completed byhuman or non-human entities.

FIG. 1 is a partially schematic illustration of an overall system 100used to generate steam. The system 100 includes one or more solarcollectors 120 operatively coupled to one or more heaters 110 (one isshown in FIG. 1 for purposes of illustration) to deliver steam to atarget 130 under the direction of a controller 140. The heater 110 cansupplement the energy provided by the solar collector 120 so as toprovide to the target 130 a relatively constant flow of steam having arelatively constant quality and/or other characteristics. In otherembodiments, the solar collector 120 and the heater 110 can operatetogether to produce steam in varying amounts and/or with varyingcharacteristics. In any of these embodiments, features of the foregoingcomponents are deliberately configured and controlled to increase theoverall efficiency and/or reduce the cost of the system 100 whencompared with conventional steam generation systems, as will bedescribed in further detail below.

Embodiments of the system 100 shown in FIG. 1 include a water flowsystem 150 that provides liquid water to the solar collector 120 and theheater 110, which in turn generate steam that is provided to the target130. In a particular embodiment, the target 130 includes a manifold 132coupled to one or more injection wells 131 used to extract oil in athermal enhanced oil recovery or thermal EOR process. In otherembodiments, the target 130 can be any of a number of suitable systemsand/or processes that use steam as an input. When the target 130 is anoilfield, the system 100 can, in particular embodiments, includemultiple combinations of solar collectors 120 and heaters 110,interconnected to a common stream distribution network and a common setof injection wells 131.

The water flow system 150 can include a water source 151 (e.g., a well,tank, reservoir, and/or combination of the foregoing elements) coupledto a network of appropriately insulated conduits 157. In general, theconduits are insulated where thermal losses are otherwise expected to besignificant, e.g., downstream of the solar collector 120 and/or theheater 110. Internal elements of the heater 110 can also be insulated.At least some elements of the solar collector 120 may not be insulated,so as not to interfere with the ability of the solar collector 120 toreceive and collect solar radiation, as will be discussed in greaterdetail later. The water source 151 delivers liquid water to an inletdevice 152 (e.g., a positive displacement pump, a multi-stageturbo-pump, or another pump, valve, or controllable device) that in turnprovides the water to an inlet conduit 153. The inlet conduit 153provides liquid water to both the solar collector 120 and the heater110. The solar collector 120 collects incident radiation from the sun,while the heater 110 generates heat from terrestrial fuel sources.

In particular embodiments, the heater 110 is an appropriately insulatedgas or other fuel-fired (e.g., fossil fuel-fired) heater that includesone or more combustion burners 111 (shown schematically as single burner111 for purposes of illustration) and directs heated combustion productsover multiple heater portions. In a representative embodiment, theheater 110 includes a first heater portion 113 and a second heaterportion 114. The first heater portion 113 can operate as a preheaterand/or economizer, and the second heater portion 114 can operate as anevaporator, radiant heater, and/or superheater. Accordingly, the burner111 directs hot combustion gases along an exhaust gas path first throughthe second heater portion 114, then through the first heater portion113, and then to a burner exhaust 112 (e.g., an exhaust stack). As aresult, the second heater portion 114 receives the highest temperatureheater gases, and the first heater portion 113 receives lowertemperature heater gases. Heated water (e.g. steam) exits the heater 110at a heater outlet 116. An exhaust gas recirculation conduit 115 canrecirculate exhaust gas from the burner exhaust 112 back to the burner111 and/or other heater elements to improve the efficiency of the heater110 by recapturing heat that would otherwise be lost to the environment.A recirculation valve 117 controls whether and to what degree theexhaust gases are recirculated. In particular embodiments, exhaust gasrecirculation is used to reduce the oxygen fraction in the combustiongas and thus slow combustion, lower peak flame temperature, and reducethe formation of oxides of nitrogen (NO_(x)), and/or gaseous pollutants.In further aspects of these embodiments, exhaust gas recirculation playsa role in maintaining adequate gas flow to maintain effective heattransfer to the heat transfer tubes within the heater 110 (e.g., tomaintain high heat transfer turbulent flow), and thus allow the burner111 to be more readily turned down. As is discussed in further detaillater, turning the burner 111 down facilitates improving and/oroptimizing the heat contributions provided by the solar collector 120and the heater 110.

The solar collector 120 can include a collector inlet manifold 121(optional) and a collector outlet manifold 122 (also optional). Thecollector inlet manifold 121 distributes incoming water throughcollection conduits 157 a (and/or other suitable solar receivers) thatreceive concentrated solar energy from one or more corresponding solarconcentrators 125. The concentrators 125 can be movable so as to trackdaily and/or seasonal shifts in the sun's relative position. The solarconcentrators 125 can include trough-shaped reflectors in someembodiments, and point-focus, linear Fresnel, and/or other concentratorsin other embodiments. The collector outlet manifold 122 collects theheated water (e.g., in liquid and/or vapor phase) and delivers theheated water to a collector outlet 124. Balance valves 123 can controlthe manner in which water is distributed throughout the solar collector120. In a particular embodiment shown in FIG. 1, the solar collector 120includes a parallel arrangement of conduits 157 a. In other embodiments,the solar collector 120 can include serial conduits and/or a combinationof parallel and serial conduits. In any of these embodiments, the steamdirected from the solar collector 120 to the collector outlet 124 can bedirected through the heater 110 and/or combined with the heated waterdirected from the heater 110 to the heater outlet 116, and then directedto a combined or overall steam outlet 156. From the overall steam outlet156, the steam is directed to the target 130. In any of theseembodiments, the outlet temperature and vapor fraction at the collectoroutlet 124 will vary widely. At some times of day (e.g., under lowerradiation conditions), the solar collector 120 will be fed lowtemperature water and deliver preheated water. At other times, theoutput of the solar collector 120 will be high-temperature, high-qualitysteam as is further discussed later with reference to FIGS. 2A-2D.

The water flow system 150, which connects the heater 110 and the solarcollector 120 to the target 130, includes a network of conduits,multiple conduit junctions 155 and valves 154 that direct the flow ofwater and steam in accordance with a number of operational modes. Forexample, the water flow system 150 can include a first junction 155 aand a corresponding first valve 154 a that selectively direct water tothe heater 110 and/or the solar collector 120. A second valve 154 bpositioned proximate to a second junction 155 b, and a third valve 154 cpositioned proximate to a third junction 155 c together direct flowexiting the first heater portion 113 to the second heater portion 114and/or the solar collector 120. A fourth valve 154 d positioned betweena fourth junction 155 d and a fifth junction 155 e directs steam fromthe collector outlet 124 to the overall steam outlet 156 and/or throughthe second heater portion 114.

The controller 140 can include a programmable logic controller (PLC), adistributed control system (DCS), and/or another suitable type ofcontrol arrangement. For example, the controller 140 can constitute aplurality of separate control elements operating either as peers or in ahierarchical arrangement, e.g., interconnected as a SCADA system viafiber optic, wired, or wireless (radio-based) communication methods. Thecontroller 140 receives inputs 141 and provides outputs 142 to directthe operation of the valves described above, the burner 111, and/orother system components. The inputs 141 can include signals receivedfrom a variety of sensors 143. Representative sensors can include afirst temperature sensor 143 a positioned to measure the watertemperature at the water source 151, a second temperature sensor 143 bpositioned to measure the steam temperature at the collector outlet 124,a third sensor 143 c positioned to measure the exhaust gas temperatureat the burner exhaust 112, and/or any of a variety of other sensors, notshown in FIG. 1 for purposes of clarity. Such sensors can include flowrate sensors (e.g., to measure steam characteristics), NO_(x) sensors,pH sensors, solar concentrator positioning sensors, solar radiationsensors, among others. The outputs 142 provided by the controller 140can direct one or more actuators 144. In addition to actuatorsassociated with each of the valves described above, the actuators 144can include a first actuator 144 a that controls the burner 111, and oneor more second actuators 144 b that control the concentrators 125 of thesolar collector 120 (e.g., the motion of the concentrators 125throughout the day and/or from season to season). These representativeactuators 144 are shown for purposes of illustration, and it will beunderstood that the controller 140 can control other actuators of theoverall system 100, which are not shown in FIG. 1 for purposes ofclarity.

The controller 140 can operate in one of more of several manners. Forexample, in a feed-forward control arrangement, the controller 140 canreceive inputs corresponding to a measured inlet water temperature andflowrate, a target outlet steam pressure and vapor fraction (steamquality), and determines the necessary enthalpy and associated fuelfiring rate for the burner 111. In a feed-back control arrangement, thecontroller 140 can receive inputs corresponding to a measured inletwater temperature and flow rate, and an outlet flow rate, and candetermine or approximate the current outlet steam quality based on oneof a variety of measured physical or more properties, e.g., a change inconductivity of the outlet liquid, a change in velocity of the flow,and/or other properties, and/or a separate measurement of liquid andvapor components of flow (e.g., via isokinetic sampling). The controller140 adjusts the burner firing rate and/or the liquid flow rateaccordingly, e.g., to return the system 100 to a particular outlet steamquality. The controller 140 can incorporate measured solar radiation anda calculated sun position to determine an expected solar energycontribution to outlet enthalpy. By including measurements of solarradiation, the controller 120 can “anticipate” changes in the enthalpyat the solar collector 120 and adjust the burner firing rates moresmoothly, adjust process controls more smoothly, and/or maintain tightercontrol of delivered steam characteristics.

In general, the thermal contribution from the solar collector 120 may beestimated and/or measured to provide feed-forward information to theburner and/or water flow rate controls. The thermal contribution fromthe solar collector 120 may be measured as inlet and outlet temperaturesand pressures, and inlet and outlet flow-rates. The outlet flow will atsome points in time be all liquid phase, and at some points in time be aliquid-vapor mixture. The steam quality at the solar collector outlet124 can be calculated and, using the James equation or other well-knownequations, may be used to calculate the enthalpy currently being addedby solar energy, which may be combined with other information togenerate an aggregate required enthalpy feed-forward signal to theburner 111

In any of the foregoing and following embodiments, the system 100 caninclude temperature sensors mounted on the conduits 157 a and/or othercomponents of the solar collector 120 to estimate the solar energycontribution, e.g., in real-time or near real-time. In the lowertemperature portion of the solar collector 120 (toward the inletmanifold 121), enthalpy is typically added as sensible heat, resultingin a rising wall temperature along the flow of liquid. The differencebetween temperature measurements taken on the conduits 157 a at severalpoints in solar collector 120 may be used to estimate the current and/orupcoming or projected solar heating intensity, and thus to estimate thetotal contribution of solar energy to the overall required enthalpy.This information, alone and/or combined with other measurements (e.g.,direct measurements of solar radiation) can be used to estimate thecorresponding required burner firing rate.

The controller 140 and associated sensors and actuators can includeother features in addition to or in lieu of the foregoing features.Representative features include feed-back based on exhaust gastemperatures, provisions for multi-fuel firing and/or handling fuel ofvarying BTU values at the burner 111, controlling the flow patternthroughout the system 100 to manage solar collector efficiency and stackgas temperature, managing/balancing optimal burner efficiency andoptimal solar collector efficiency, controlling a flow of waste heatfrom an enclosure surrounding the solar collector to the burner inlet,and/or dumping heat at the solar collector 120, e.g., when the energyprovided by the solar collector 120 exceeds the energy required by thetarget 130. The extent to which heat is dumped can depend on factorsthat include the local environmental conditions and/or the size of thesolar collector 120 relative to the demands of the target 130 and therelative capacity of the heater 110. The foregoing control arrangementscan include calculations that account for and/or make use of systemcharacteristics, including that the coldest water provided to the solarcollector 120 delivers the highest BTU output from the solar collector,and the lowest exhaust gas temperature delivers highest BTU output fromburner 111.

FIGS. 2A-2D illustrate configurations of the system 100 shown in FIG. 1in accordance with representative operational modes directed by thecontroller 140. In particular embodiments, the operational modes can becorrelated with the time of day during which the system 100 operates. Inother embodiments, the operational modes can be tied to other variables,including seasonal variables, and in still further embodiments, thesystem 100 can be configured to operate in modes other than thoseexpressly shown in FIGS. 2A-2D and described below. In general, thecontroller 140 directs smooth, gradual transitions from one mode toanother, e.g., to reduce or eliminate thermal shock, sudden fluid flowchanges, and/or other potentially undesirable effects.

FIG. 2A illustrates the system 100 configured in a representative firstoperational mode that, in at least some embodiments, corresponds tonight-time operation. In this embodiment, the first valve 154 a, thesecond valve 154 b, and the fourth 154 d are closed, and the third valve154 c is open. Accordingly, water travels from the water source 151 tothe steam outlet 156 via the heater 110, and bypasses the solarcollector 120, which is not active at night. In this mode, the waterflow (indicated by arrows F) passes through the first heater portion113, e.g., for preheating, then to the second heater portion 114, e.g.,for further heating and phase change to steam, and then to the steamoutlet 156 for delivery for to the target 130. The lack of flow throughthe solar collector 120 prevents the water in the conduits from flowingbackwards to the solar collector 120 at the third junction 155 c.Because the solar collector 120 is not active during the firstoperational mode, the burner 111 is typically operating at a high rate.The exhaust gas recirculation valve 117 can be opened or closed,depending upon the exhaust gas temperature and/or other factors. Ingeneral, the exhaust gas recirculation conduit 115 can be active (e.g.,with the gas recirculation valve 117 open) when the energy cost forblowing or pumping the recirculating exhaust is less than the energyloss due to dumping high temperature exhaust gas to the environmentand/or to control (reduce) emissions of NO_(x) and/or other gases.

FIG. 2B illustrates the system 100 configured in a second operationalmode, e.g., corresponding to a low incident radiation level at the solarcollector 120. For example, the second operational mode can correspondto early morning, after sunrise. In this mode, the first valve 154 a ispartially opened. Accordingly, the flow of water indicated by arrow Fsplits at the first junction 155 a into a first flow (indicated by arrowA) that is directed to the first heater portion 113, and a second flow(indicated by arrow B) that is directed to the solar collector 120. Thefirst flow passes through the first heater portion 113 to the thirdjunction 155 c. The second flow is directed to the solar collector 120.At the solar collector 120, the solar concentrators 125 are positionedto focus the incident solar radiation to heat the water passing throughthe conduits 157 a. The heated water passes through the collector outlet124 and combines with the first flow at the third junction 155 c toproduce a combined flow (indicated by arrow C). The combined flow passesthrough the second heater portion 114 and is provided to the steamoutlet 156 for delivery to the target 130. For purposes of illustration,the first valve 154 a is shown partially open and the third valve 154 cis shown fully open in FIG. 2B. In other embodiments, the relativepositions of these valves can be reversed, or both valves can bepartially open, depending upon factors that can include, but are notlimited to, the relative pressure drops through the heater 110 and thesolar collector 120, respectively.

During the course of the early morning, as solar radiation received atthe solar collector 120 intensifies, more and more flow is directed tothe solar collector 120, and less flow is directed to the first heaterportion 113, by further opening the first valve 154 a. In addition, asthe heat output produced by the solar collector 120 increases (e.g., inthe form of sensible heat and/or latent heat), the burner 111 is turneddown to reduce unnecessary fuel consumption. In typical arrangements,the burner 111 has different performance characteristics at differentoperating points, which is accounted for in determining how far to turnthe burner 111 down. In general, it is more efficient to turn the burner111 down than to stop and restart the burner 111. As the flow of waterthrough the first heater portion 113 is reduced, the temperature at theburner exhaust 112 may rise, even though the burner 111 is turned down,due to the reduced amount of exhaust gas heat transferred to the waterin the first heater portion 113. The increased burner exhausttemperature can reduce the net efficiency of the fuel-fired portion ofthe system (e.g., the heater 110), even while the exhaust gasrecirculation conduit 115 is active. To reduce or eliminate suchinefficiencies, the system 100 can move to a third operational mode,described below with reference to FIG. 2C.

Referring now to 2C, in the third operational mode, the first valve 154a and the third valve 154 c are closed, and the second valve 154 b isopened. Accordingly, the flow of water indicated by arrows F passesfirst through the first heater portion 113, then to the solar collector120, and then to the second heater portion 114 before being delivered tothe steam outlet 156 and the target 130. In this embodiment, the solarcollector 120 adds a significant amount of heat to the water, producinglow to medium quality steam, and the second heater portion 114 increasesthe quality of the steam to a level suitable for the target 130. Typicalquality levels at the steam outlet 156 range from about 65% to about80%, but can vary outside this range in particular embodiments. Typicalpressures can be from about 400 psi to about 2800 psi, although thesevalues, too, may be different in other embodiments. In any of theseembodiments, the system 100 can quickly respond to intermittent changesin the heat output provided by solar collector 120. For example, if acloud passes between the solar collector 120 and the sun, temporarilyreducing the heat output of the solar collector 120, the burner 111 canbe quickly turned up to supplement the heat provided by the solarcollector 120. The burner 111 can be quickly turned down again when thecloud cover has passed. If the sun remains behind a cloud for extendedperiods of time, the system flows can be readjusted in a gradual mannerto rebalance the system 100, e.g., by reverting to the secondoperational mode (FIG. 2B) or the first operational mode (FIG. 2A).

During at least some seasons (e.g., midsummer), and at least some hours(e.g., around midday), the system 100 can enter a forth operational mode(shown in FIG. 2D) corresponding to a peak level of incident radiationreceived at the solar collector 120. In this mode, the fourth valve 154d can be at least partially opened to produce a first flow (indicated byarrows A) passing through the second heater portion 114, and a secondflow (indicated by arrows B) passing directly from the solar collector120 to the fifth junction 155 e. At the fifth junction 155 e, the twoflows combine to produce a combined flow (indicated by arrow C) at thesteam outlet 156. In this mode, the burner 111 can be further turneddown, while a sufficient flow of water is directed through both thefirst heater portion 113 and second heater portion 114 to prevent theseportions from drying out or otherwise degrading due to low flowconditions. In at least some cases, the peak steam production capacityof the solar collector 120 can exceed the input needs of the target 130.In such instances, excess steam or heat can be diverted to other uses(e.g., thermal storage and/or thermoelectric power generation via asteam-powered turbine generator or other suitable device), or some ofthe solar concentrators 125 can be moved to an off-focus position toreduce excess output.

Suitable thermal storage techniques and systems include (a) passing theheated water through a heat exchanger (e.g., with liquid salt, thermaloil, or another suitable composition as a working fluid in thermal, butnot fluid, communication with the water); (b) embedding steam conduitsin concrete or another thermal storage medium, including in some cases,a phase change material; and/or (c) using a wet steam accumulator. Inany of these embodiments, the excess steam production can result from anoutput by the solar collector 120 that exceeds the output of the heater110 alone. In some cases, this excess steam capacity may be used by thetarget 130 (e.g., an oilfield) as oil production expands over the lifeof a typical thermal EOR project. Suitable thermal storage media andassociated techniques are described further below with reference toFIGS. 7A-7E.

The injection wells 131 can accept steam flow at a rate determined bycharacteristics including the diameter and depth of the injection wells,their downhole completions, and by characteristics of the formationincluding downhole pressure and injectivity (resistance to flow). Assteam injection and oil production proceed during the life of a thermalrecovery project, downhole pressure and injectivity change. Typicallydownhole pressure is reduced in an effort to make steam more effective,and injectivity rises as the areas of the formation surrounding theinjector well heat up and experience other physical changes associatedwith steam and oil production. Accordingly, the peak rate of steam flowdown an individual injector well for a given wellhead steam pressurewill typically rise from the initial value when the well is firstcompleted and steam injection commences. Accordingly, asolar-versus-fuel-fired steam balance may be planned to change over theearly life of a steamflood project, because higher peak rates becomepossible and as a result a higher total amount of steam may be deliveredduring daylight hours (when solar steam generation is operating), andthe burner 111 may be turned down more deeply. Similarly, aninstallation of steam generation may be planned which during the earlylife of the field requires dumping solar energy during peak-radiationperiods, whereas later during the life cycle, less or no dumping isrequired as the system can accommodate higher peak flows.

In any of these embodiments, after the peak incident radiation periodhas passed, the system 100 can revert to the third mode shown in FIG.2C. As the incident sunlight further wanes toward the end of the day,the system 100 can then revert to the second mode shown in FIG. 2B, andas night falls, can revert to the first mode shown in FIG. 2A. As thesystem 100 transitions to night time operation (the first operationalmode), the water passing through the solar collector 120 absorbsresidual heat from the solar collector 120. Accordingly, by the time thesolar collector 120 is isolated from the water flow passing through theheater 110, the residual heat remaining in the solar collector has beeneffectively absorbed by the water and converted to steam, thus reducingor eliminating what would otherwise be radiation losses from the solarcollector 120 at night.

One feature of an embodiment of the system 100 described above is thatit can include a single heater that can be coupled to the solarcollector 120 in parallel or in series, depending on the operationalmode. For example, in the first operational mode shown in FIG. 2A, thefirst and second heater portions 113, 114 operate in series with eachother, and the solar collector 120 is excluded from the circuit. In thesecond operational mode shown in FIG. 2B, the first heater portion 113of the heater operates in parallel with the solar collector 120, and thesecond heater portion 114 operates in series with the solar collector120. In the third operational mode shown in FIG. 2C, both the first andsecond heater portions 113, 114 operate in series with the solarcollector 120, with the first heater portion 113 coupled upstream of thesolar collector 120, and the second heater portion 114 coupleddownstream of the solar collector 120. In the fourth operational modeshown in FIG. 2D, the first heater portion 113 operates in series withthe solar collector 120, and the second heater portion 114 operates inseries with only a part of the flow exiting the solar collector 120.Accordingly, the controller can control the flow of water through thesystem, and can direct the water through the solar collector 120 and theheater 110 in accordance with different sequences. In particularembodiments, one portion of the flow is directed through the solarcollector and the fuel-fired heater in accordance with a first sequence,and the same (or a different) portion of the flow is directed throughthe solar collector and the fuel-fired heater in accordance with asecond, different, sequence.

As a result of the forgoing multiple operational modes, the outputprovided by the solar collector 120 can be improved (e.g., optimized) totake best advantage of the incident solar radiation, and the outputprovided by the heater 110 can be improved (e.g., optimized) tosupplement the output provided by the solar collector 120, as needed,and to improve or optimize the efficiency with which the heater 110operates. This is unlike typical existing arrangements which includeheaters arranged only in series or only in parallel.

As noted above, the system 100 can operate in at least the foregoingoperational modes with only a single heater 110 in addition to the solarcollector 120. The single heater can include multiple heater conduitsand flow paths, multiple burners and multiple heater portions, but in atleast some embodiments, hot gas flow from the burner(s) passes seriallyover at least two water-carrying portions, (e.g., the first portion 113and the second portion 114), each of which can carry anindividually-controllable flow of water. This arrangement can reduce thenumber of heaters used to supplement the solar collector 120, while atthe same time providing the flexibility to increase efficiencies in boththe solar collector 120 and the heater 110 as conditions change.

Another advantage of the foregoing arrangements is that embodiments ofthe system 100 do not include a separator that separates low qualitysteam and/or water from high quality steam. Instead, any steam that mayhave less than the desired quality level passes through the heater 110to provide sufficient control over the quality of steam provided to thetarget 130.

Another feature of at least some of the foregoing embodiments is thatthey can take advantage of the ability to turn down the burner 111, thusreducing fuel consumption that may be unnecessary in light of the heatprovided by the solar collector 120. Accordingly, it is desirable toinclude a burner 111 that has a deep turndown ratio so that the highesttotal fraction of solar steam may be provided to the target 130. Typicalfuel-fired burners 111 have a limited turndown ratio. These limits canbe due to the operational characteristics of the burner 111 itself. Forexample, the burner 111 will typically deliver a stable, clean(low-emission) flame only within a designed operating range. Theturndown ratio of the burner 111 may also depend on whether the burner111 is capable of being operated with more than one fuel, for example,diesel or natural gas, or gas with a varying range of heating values.The turndown ratio of the burner 111 may also depend on whether featuressuch as exhaust gas recirculation are used to reduce the emissions ofcertain combustion products, such as nitrogen oxides (NO_(x)). Theability to turn down the burner 111 is also restricted by the need tomaintain effective mixing flow across the heat exchanger surfaces withinthe heater 110 (e.g., at the first heater position 113 and/or the secondheater position 114). If an inadequate air flow is provided to theheater portions 113, 114, the temperature at the burner exhaust 112 willincrease, increasing the amount of heat dumped to the environment. Ifthe flow of water through the heater 110 is inadequate, the heatexchanger tubing in the first heater portion 113 and/or the secondheater portion 114 may overheat, causing the tubes to rupture and/or thetemperatures at the burner exhaust 112 to increase, again increasingmaintenance costs and reducing overall efficiency.

The foregoing turndown limitations can be accommodated with the systemconfiguration shown in FIG. 1. For example, in both the second and thirdoperational modes (FIGS. 2B and 2C, respectively), the full flow ofwater is routed through the second heater portion 114, reducing oreliminating the likelihood that the flow of water through the secondheater portion 114 will be inadequate. In the first, third and fourthoperational modes (FIGS. 2A, 2C and 2D, respectively), the full flow ofwater is directed through the first heater portion 113, reducing oreliminating the likelihood for inadequate flow during these modes.During the second mode, the first valve 154 a can be monitored so as tocontinue providing adequate flow to the first heater portion 113 as anincreasing amount of flow is directed to the solar collector 120.Because the first heater 113 operates at lower temperatures than thesecond heater portion 114, the first heater 113 can tolerate a greaterreduction in flow. The system 100 can be sized so that even when someflow bypasses the second heater portion 114 in the fourth operationalmode, an adequate flow is provided. In this mode, the total flow throughthe system can be increased relative to the flow provided during thefirst, second, and third operational modes.

Another feature of embodiments of the system described above include asingle (or in some embodiments, an integrated) controller arrangementthat monitors and controls both the solar collection 120 and the heater110 to produce the desired flow rate and steam quality delivered to thetarget 130. This can be a peer relationship established between twoseparate controllers, or a hierarchical relationship where, e.g., amaster controller communicates with a solar controller and a heatercontroller, or a single controller provides such communication. This isunlike conventional arrangements for which these two control andmonitoring functions are separated, which can be more costly and lessefficient to implement than the presently described technology.Embodiments of the present systems can also reduce the number of overallsystem components because several components are shared by the solarcollector 120 and the heater 110. These components include a singleinlet device 152 that directs flow to both the solar collector 120 andthe heater 110, and a single overall steam outlet 156.

Still another feature of embodiments of the system described above isthat flow rates through the solar collector 120 can be relatively highat both low and high incident radiation conditions. At low incidentradiation conditions, the second heater portion 114 supplements, asneeded, heat produced by the solar collector 120. At high incidentradiation conditions, the high flow rate reduces excessively hightemperatures at the solar collector 120, which would otherwise increaseradiation losses. Such losses can be significant, as these areproportional to temperature raised to the fourth power. In particular,elements of the solar collector 120, e.g., the collection conduits orother receivers, are generally uninsulated so as to facilitate absorbingsolar radiation. As the temperature of the receivers increases, lossesincrease at a very high rate (as T⁴). Accordingly, it may be desirablein at least some embodiments to keep the solar collector 120 (orportions of the solar collector 120) at lower rather than highertemperatures. This can be accomplished, for example, by having a greaterportion of the solar collector 120 elevate the water toward thevaporization temperature, and a lesser portion of the solar collector120 evaporate the water (change its phase) and/or elevate the vaporbeyond the vapor transition temperature. Aspects of the system 100 allowthe operator to optimize, for each radiation condition, the efficiencyof the solar collector 120 and the heater 110. Flow is apportionedbetween the first portion 113 of the heater 110 and the solar collector120 based on consideration of the simultaneous efficiency of each forthe currently available solar radiation.

FIG. 3 is a partially schematic illustration of a system 300 that caninclude fewer valves than the system 100 described above, in accordancewith another embodiment of the present technology. The system 300includes several features similar or identical to the correspondingfeatures shown in FIG. 1, including a water source 151, a heater 110, asolar collector 120, a controller 140, and a target 130. The system 300further includes a first valve 354 a but may not include correspondingsecond, third, and fourth valves such as those shown in FIG. 1. Instead,the first valve 354 a can be selectively adjusted to (a) direct all thewater from the water source 151 through the heater 110 (and no waterthrough the solar collector 120), or (b) a first portion of the waterthrough the heater 110 (in particular, the first portion 113 of theheater 110) and another portion of the water through the solar collectorand the second portion 114 of the heater 110, or (c) all the water fromthe water source 151 through the solar collector 120 and the secondportion 114 of the heater 110. This arrangement provides for betterefficiency than an arrangement in which the heater 110 and the solarcollector 120 are in a fixed parallel configuration. For example, thisarrangement can provide improved efficiency during the early part of theday, as discussed above with reference to FIG. 2B, and can isolate thesolar collector 120 and recapture residual heat from the solar collector120, in a manner generally similar to that discussed above withreference to FIG. 2A. In addition, with the arrangement shown in FIG. 1,the arrangement shown in FIG. 3 does not include a separator.

FIG. 4 is a partially schematic illustration of a system 400 thatincludes a heat recovery steam generator 470, in accordance with stillanother embodiment of the present technology. The heat recovery steamgenerator 470 can include a combustion turbine 462 that provides hotexhaust gases to a heater 410. The exhaust gases can optionally befurther heated via a supplemental heater 463 (e.g., a duct heater). Theheater 410 can include a first portion 413 (e.g., an economizer)positioned downstream in the exhaust gas flow path from a second portion414 (e.g., a superheat portion). The heater 410 can further include anevaporator 460 positioned upstream of the first portion 413. In otherembodiments the evaporator can have other locations, e.g., upstream ofthe second portion 414. A steam drum 461 is positioned to collect liquidand/or vapor phase water and separate the vapor phase water from theliquid phase water. A water source 551 directs water to the solarcollector 120 and/or the heat recovery steam generator 470, depending,for example, upon the settings of a first valve 454 a, a second valve454 b, and a third valve 454 c. The valves and other features of thesystem 400 are under the control of a controller 440 that receivesinputs 141 and directs outputs 142.

In operation, the water source 551 can direct water to the solarcollector 120 alone, to the heat recovery generator 470 alone, or toboth the solar collector 120 and the heat recovery generator 470,depending upon the position of the first valve 454 a. Water directed tothe solar collector 120 (as indicated by arrow A) returns to the steamdrum 461, as indicated by arrow I. Water directed to the heat recoverysteam generator 470, as indicated by arrow B, passes through the firstportion 413 and then to the evaporator 460 (as indicated by arrow C),and/or around the evaporator 460 (as indicated by arrow D). Waterdirected to the evaporator 460 is evaporated and passes to the steamdrum 461 as indicated by arrow E. Condensate from the steam drum 461returns to the evaporator 460 as indicated by arrow F. Vapor passes fromthe steam drum 461 to the second portion 414 as indicated by arrow G.After being heated at the second portion 414, the vapor can optionallypass through an attemperator 464 that sprays water received from thewater source 551 into the steam flow to manage the temperature of thesteam, which is then directed to the target 130.

Water bypassing the evaporator 460 (as indicated by arrow D) can passdirectly to the steam drum 461, and/or to the solar collector 120,depending upon the position of third valve 454 c.

In the embodiment shown in FIG. 4, the system 400 includes a heatrecovery steam generator 470 coupled to the solar collector 120 in amanner that achieves at least some of the features and advantagesdescribed above. For example, the system 400 can selectively directwater to the solar collector 120 and/or the heater 410. The system canrecapture heat from the solar collector 120 during and/or approachingnight time operation, in a manner generally similar to that describedabove with reference to FIGS. 2A-3.

One feature of many of the embodiments described above is that theembodiments can include a heater (e.g., a boiler) with a pre-heatsection, in combination with a solar concentrator, both of which receivethe same working fluid throughout. This arrangement can be configured inaccordance with a number of operational modes to enhance the overallefficiency of the system. In particular, the systems can be optimized toefficiently provide a target quantity and quality of steam, accountingfor a wide variety of factors that include radiative losses at the solarcollector 120, and fuel use at the heater. The thermal efficiency of thesolar collector is loosely determined by the temperature of the receiverelements at the solar collector, the inlet temperature of the water atthe solar collector, the flow rate of water through the solar collector,the collected radiation. The efficiency of the heater can be defined bya variety of parameters, including the inlet water temperature, flowrate, exhaust gas temperature, and the fuel burn rate. The overallsystem can be configured to increase the efficiency of, and/or optimizethe operation of, the steam generation process by adjusting theinteractions between the heater and the solar collector. This flexiblearrangement provides for efficient operation despite varyingenvironmental conditions.

FIG. 5 is a representative graph illustrating the heat contributionprovided by a representative heater and a solar collector, along withthe cumulative heat provided by both, as a function of hours during atypical day of operation. As shown in FIG. 5, the system can be tailoredto provide a generally constant cumulative heat output by varying theheat contribution provided by the heater as the heat contributionprovided by the solar collector increases and decreases during thecourse of the day.

FIG. 5 illustrates general variations in the heat contribution providedby a solar collector. FIG. 6 illustrates with more granularity thevariation in the radiation incident on the solar collector duringtypical operation. As is evidenced from FIG. 6, the incident solarradiation can vary significantly, and can vary quickly. As discussedabove, embodiments of the present technology include a heateroperatively integrated with the solar collector to account for suchvariations while still providing a uniform quality and quantity ofsteam, as may be required by the target user.

Particular embodiments of the systems described above experiencetime-varying average temperatures at the solar collector, which aremanaged in such a manner as to optimize (or at least improve) totalsteam output as a function of fuel firing. Based on expected currentenergy production (e.g., based on radiation/time of day), the systemscan apportion boiler feedwater flow rates and flow patterns in themanners described above. At least some conventional systems focus onadjusting the flow rates in a solar boiler so as to achieve a particularoutlet steam temperature or outlet steam condition, holding that steamcondition a constant. By contrast, embodiments of the present technologyfocus on achieving the greatest combined efficiency, integrated over afuel-fired heater and solar steam generator. The controller canapportion flow rate and flow patterns so as to optimize, or at leastimprove, the total steam produced by the combination—balancingefficiency losses in the fuel-fired boiler due to effects of reducedliquid flow causing higher stack temperatures, against efficiency lossesat the solar collector, which vary strongly with the temperature of thecollector. The controller manages the combined system in such a way(e.g., via the modes described above) that during lower-radiationperiods, the solar field collector is operating at lower averagetemperatures than a parallel arrangement would provide. Embodiments ofthe systems can monitor and respond to both the water inlet temperatureand the system outlet temperature, as well as the outlet steam quality.As the efficiency of a solar collector varies strongly with its receivertemperature (since radiation losses vary as T⁴), lowering thetemperature (e.g., the average temperature and/or the peak temperature)can significantly improve the overall system efficiency.

The temperature of the steam in particular embodiments of the presenttechnology is ultimately determined by the system outlet pressure, andin particular, the saturation temperature for that pressure. In a puresolar boiler, inlet water flows through the solar collector, rising intemperature until it begins to boil. As boiling water flows through thetubes, its temperature changes little as more thermal energy is added,driving the phase change from liquid to vapor phase until enough energyhas been added to complete the conversion to steam. However, in atypical operating mode for embodiments of the presently disclosedsystems, the outlet quality is less than 100% steam. As a result, thetemperature profile of the solar collector is determined by thecurrently available solar radiation, the inlet feedwater temperature,and the inlet flow rate. Higher flow rates will reduce the outlet steamquality (perhaps to 0) and will push the boiling point in the collectorfurther downstream. Lower inlet water temperatures will have the sameeffect. However, flow diversion from away from the heater and to thesolar collector will raise the exhaust gas temperature at the heater andreduce its thermal efficiency proportionally. Aspects of the presenttechnology are direct to balancing these countervailing effects. Foreach radiation condition, for each size of solar collector andfuel-fired boiler, with their respective efficiency curves, there existsan optimum operating point at which the combined efficiency of the twois maximized or at least improved. The controller logic can beconfigured to achieve or approximately achieve this operating point.

FIGS. 7A-7E illustrate systems in accordance with still furtherembodiments of the present technology. These systems include thermalstorage functions and/or close proximity between the solar collector andthe heater, with both features configured to increase the overallefficiency with which steam is generated and provided to an oil field orother target site. Several features of the systems shown in FIGS. 7A-7Eare common to those described above with reference to FIGS. 2A-2E andare therefore shown and/or described below with less detail. Suchfeatures will typically operate in manners generally similar to thosedescribed above with reference to FIGS. 2A-2E.

FIG. 7A illustrates a system 700 a that includes a solar collector 120,a heater 110, and a water flow system 150 coupled together to providesteam at an overall steam outlet 156, which is directed to a target 130.The water flow system 150 can include one source of multiple (e.g.,distributed) sources. Individual sources can operate autonomously or canbe interconnected, depending upon the embodiment. In addition, thesystem 700 a can include a storage medium 770, e.g. a tank or othersuitable medium, that stores water or another working fluid in such amanner as to preserve the thermal energy of the working fluid for lateruse. Accordingly, the storage medium 770 can include insulation and/orother features that reduce thermal losses.

The system 700 a can include a series of pumps 152 a-152 d (referred tocollectively as pumps 152) and valves 754 a-754 g (referred tocollectively as valves 754) that direct the water or other working fluidin a variety of manners depending upon the operational mode. Forexample, as shown in FIG. 7A, the system 700 a can include a watersource 151 that provides water to a first pump 152 a. The first pump 152a can direct the water to the heater 110 via a first valve 754 a and asecond pump 152 b, or can direct the water to the storage medium 770 viaa second valve 754 b. A third valve 754 c controls whether the waterdirected from the water source 151 is provided to the storage medium 770or to the solar collector 120. Water released from the storage medium770 can be directed via a third pump 152 c to the solar collector 120 orto the heater 110, depending upon the settings of a fourth valve 754 dand a fifth valve 754 e. Water directed to the solar collector 120 canbe further pressurized by a fourth pump 152 d. The positions of thevalves 754 shown in FIGS. 7A-7E are illustrative only—the valves 754 canassume positions other than those shown in the Figures depending uponthe operational mode of the system at any given point in time.

Water (e.g., in liquid and/or vapor form) exiting the solar collector120 can be directed to the overall steam outlet 156, or to the storagemedium 770, depending upon the setting of a sixth valve 754 f and aseventh valve 754 g. For example, when the solar collector 120 producesrelatively low quality steam (or no steam), e.g., when the solarcollector 120 is starting up, shutting down, and/or is subject to low orno solar insolation (e.g., due to cloud cover), the water/steam can bedirected (e.g., diverted) to the storage medium 770. This mode ofoperation can be entered as a function of time of day (e.g., morning orevening) and/or other parameters (e.g., insolation or steam quality). Asthe quality of the steam put out by the solar collector 120 increases,the output (or a greater fraction of the output) is provided to theoverall steam outlet 156. Accordingly, heat received at the solarcollector can be directed to the overall steam outlet 156 (and fromthere to an oil field or other target) during a first operational mode,and can be directed to the thermal storage medium 770 during a secondoperational mode.

In one mode of operation, the solar collector 120 and the heater 110 canoperate together to provide steam to the overall steam outlet 156, in amanner generally similar to that described above with reference to FIG.2D, with the sixth valve 754 f opened. During nighttime operation, orother phases when the solar collector 120 produces no steam or aninsufficient quantity of steam, the sixth valve 754 f can be closed toisolate the solar collector 120 from the heater 110, the overall steamoutlet 156, and the target 130. In at least some embodiments, the waterremaining in the solar collector 120 and associated conduits can bedirected through a steam storage line 771 to the storage medium 770, soas to avoid losing the heat contained in this working fluid. During asubsequent start up process, the solar collector 120 can remain isolatedfrom the overall steam outlet 156 and the heater 110 to avoid the needfor a “blow down” process (and associated “hammer” effects) that may becaused by mixing high temperature steam created by the heater 110 withwater or a mixture of water and low quality steam produced during thestartup of the solar collector 120. Once the quality of the steamproduced by the solar collector 120 exceeds a threshold value (e.g. 80%,90% or 95% depending upon the embodiment), the sixth valve 754 f can beopened to allow the steam provided by the solar collector 120 tosupplement and/or replace the steam provided by the heater 110.

The solar collector 120 can be linked to the heater 110 with a firstconnecting line 772, and the overall steam outlet can be linked to thetarget 130 with a second connecting line 776. During the day, the secondconnecting line 776 remains hot due to the steam provided by the heater110 and/or the solar collector 120. At night, the second connecting line776 remains hot due to the steam provided by the heater 110, but thefirst connecting line 772 cools, and must be re-heated when the solarcollector 120 restarts the next day. Accordingly, in a particularembodiment, a first length L1 of the first connecting line 772 betweenthe solar collector outlet 124 and the heater outlet 116 can be reduced,so as to reduce or minimize the thermal losses that may result from along first connecting line 772 that is exposed to radiation lossesduring the night. For example, in at least some existing conventionalarrangements, the first length L1 of the first connecting line 772 canexceed 1,000 meters, 5,000 meters, or even 10,000 meters. In particularembodiments of the presently disclosed technology, the length of thefirst connecting line 772 can be reduced, e.g. to a value of less than1,000 meters and in further particular embodiments, less than 500meters, less than 250 meters, less than 100 meters, less than 50 meters,or less than 10 meters. By co-locating the heater 110 and the solarcollector 120 and reducing the first length L1, the overall systemthermal losses can accordingly be reduced.

In addition to (or in lieu of) shortening the length L1 of the firstconnecting line 772, the system 700 a can be configured to keep one ormore portions of the first connecting line 772 hot, even when the solarcollector 120 is inactive. For example, the first connecting line 772can include an upstream portion 772 a and a downstream portion 772 b. Bysloping the first connecting portion 772 (to provide gravity-drivencondensate flow) and/or by properly adjusting the sixth, seventh, and/orother valves, the system can maintain the downstream portion 772 band/or the upstream portion 772 a at a pressure and temperature thatmaintain the flow and/or presence of steam.

Because the second connecting line 776 is generally heated continuously,it may be less important to reduce the second length L2 of this line. Inparticular embodiments, the second connecting line 776 can be insulatedto reduce thermal losses. In some embodiments, an additional heater 110can be positioned along the second connecting line 776 to offset thermallosses. In any of the foregoing embodiments, the heaters 110 can bedistributed so as to keep the lines hot, thus delivering increased highquality steam to the target 130.

FIG. 7B illustrates a particular embodiment in which an oil field isprovided with steam injection, that is in turn provided from multiplesources located around the oil field. The multiple sources can includeone or more solar fields (each of which can include one or more solarcollectors 120) coupled in close proximity to a heater and/or heatstorage medium (e.g., a heater 110 and storage medium 770, as shown inFIG. 7A). Individual solar fields can have corresponding dedicatedstorage media (e.g., storage tanks) and/or dedicated water supplies. Inother embodiments, the storage media and/or water supplies cam beinterconnected. In particular embodiments, the overall system can alsoinclude a central heater source that supplements heat provided byindividual pairings of heaters and solar fields. The central heatersource can also include mobile heaters that may be moved to differentlocations that require heat as the steam requirements of the oil fieldshift or change. As shown in FIG. 7B, at least some of the solar fieldscan be located well beyond the initial edge of the injection zone, toallow for expansion of the oil field injection operations withoutinterfering with the solar collection process. For example, newinjection wells may be added between initial injection wells and one ormore of the solar fields because the solar fields are positioned farenough away from the initial injection wells to allow for additionalinjection wells.

FIG. 7C illustrates another embodiment of an overall system 700 c thatincludes several features similar to those described above withreference to FIG. 7A. In addition, the system 700 c can include aseparator 775 that separates higher quality steam from lower qualitysteam and/or water. Accordingly, steam or a liquid/steam mixture exitingthe solar collector 120 enters the separator 775, with steam directed tothe sixth valve 754 f and the overall steam outlet 156. Water isdirected from the separator 775 to the seventh valve 754 g for deliveryto the storage medium 770. In addition, the separator 775 itself canserve as a storage medium and can accordingly be insulated and/orotherwise configured to reduce thermal losses. In a particularembodiment, the system 700 c can include a heat exchanger 776 (e.g., acounterflow heat exchanger) that transfers heat from the water separatedat the separator 775 to water entering the solar collector 120. In theillustrated embodiment, the heat exchanger 776 is positioned upstream ofthe fourth pump 152 d. In other embodiments, the heat exchanger 776 canbe positioned downstream of the fourth pump 152 d. In any of theseembodiments, the water removed from the separator 775 and directedthrough the heat exchanger 776 can then be directed to the storagemedium 770, as discussed above. In still further embodiments, forexample, when the water includes an unacceptably high level of sediment,minerals, and/or other particulates, the water removed from theseparator 775 can instead be directed to a filtration system, or simplydumped, as indicated in dashed lines in FIG. 7C.

The system 700 c can also include an eighth valve 754 h and a ninthvalve 754 i. The eighth valve 754 h can optionally direct steam to thestorage medium 770. The ninth valve 754 i can be used to selectively addwater back to the flow of steam produced at the separator 775 to controlthe quality of the steam.

FIG. 7D illustrates a system 700 d configured in accordance with stillanother embodiment of the present disclosure. In this embodiment, thesystem 700 d includes a separator 775 and a storage medium 770, asdiscussed above with reference to FIG. 7C. In addition, the system 700 dcan include a valving arrangement, generally similar to that describedabove with reference to FIGS. 2A-2D, which allows water to beselectively directed the first heater portion 113 and/or the secondheater portion 114 of the heater 110. Accordingly, the system 700 d caninclude a tenth valve 754 j, an eleventh valve 754 k, and a twelfthvalve 754 l. Each of these valves can be adjusted to direct waterthrough only the first heater portion 113 (e.g., by opening the tenthvalve 754 j and closing the eleventh valve 754 k and the twelfth valve754 l) or through both the first heater portion 113 and the secondheater portion 114. For example, with the tenth valve 754 j open and theeleventh valve 754 k closed, water is pre-heated at the first heaterportion 113 before entering the solar collector 120. By opening thetwelfth valve 754 l, water collected at the separator 775 can bedirected to the second heater portion 114 so as to be converted tosteam. Accordingly, the twelfth valve 754 l can be operated to directwater removed from the separator 775 into the heater 110, rather than tothe storage medium 770. In this mode of operation, the seventh valve 754g is closed at the same time the twelfth valve 754 l is opened.

FIG. 7E illustrates a system 700 e configured in accordance with yetanother embodiment of the present technology. In this embodiment, thesystem 700 e includes features generally similar to those discussedabove with reference to FIG. 7D, and, in addition, include a thirteenthvalve 754 m. The thirteenth valve 754 m, in combination with the twelfthvalve 754 l, allow water removed from the separator 775 to be directedto either the first heater portion 113 or the second heater portion 114.The settings of these valves can be determined by the controller 140,and the process for opening and/or closing these and/or any other valvesof the system can be based upon the relative temperatures and pressuresof the water in the system, the extent to which the water requiresfurther heat, the demands of the target 130 and/or other factors. Forexample, if the water exiting the separator 775 is at a temperaturebelow that of the first heater portion 113, it can be directed to thefirst heater portion 113 before being directed to the second heaterportion 114. If the water exiting the separator 775 has a temperatureabove that of the first heater portion 113 and below that of the secondheater portion 114, it can be directed to the second heater portion 114,bypassing the first heater portion 113.

Embodiments of the systems described above with reference to FIGS. 7A-7Einclude features that can provide one or more of several advantages overconventional systems. For example, at least some of the embodiments ofthe foregoing systems include a storage medium. The storage medium canrecover heat that is present in the solar collector 120 after shutdownand/or during periods of low output, and can accordingly eliminate or atleast reduce the losses associated with simply allowing this heat toexit the system via radiation, convection, and/or conduction.

Another feature of at least some of the foregoing embodiments is thatthe heater 110 can be operated to provide a continuous flow of steamovernight. In some embodiments, the amount of steam produced by theheater at night is sufficient to meet the requirements of the target130, e.g., to maintain an oil field injection process at full capacity.In other embodiments, the heater 110 can operate at night simply tomaintain a threshold flow of steam through the system. This arrangementcan eliminate the thermal cycling that would otherwise be experienced bythese portions of the system, and can eliminate water accumulation inthe lines to which it is connected. This in turn can reduce operatingrisks and startup delays, and can reduce maintenance costs. When used inthis latter mode (e.g., to provide a threshold level of steam that maybe below typical operational requirements), the heater can beappropriately sized to meet this functional requirement. In particular,the heater can be smaller than a heater sized to produce the full steamoutput required by the target 130, and/or can be configured to operateefficiently at low output levels when simply maintaining the thresholdlevel of steam rather than a fully operational level of steam.

Another feature of at least some of the foregoing embodiments includesmaintaining steam in a greater fraction of the lines associated withsteam production, despite the cyclic nature of the solar collector. Thisapproach can reduce the likelihood for line blow down. For example, asdiscussed above, the conduits associated with the solar generator can beisolated from the rest of the system during startup to reduce oreliminate contact between water and high quality steam, until the solargenerator produces steam at a high enough quality to be reconnected withthe rest of the system. This arrangement can reduce the waste steamassociated with line blow down, reduce lost steam production periodsassociated with line blow down, and/or reduce the maintenance costsassociated with thermal cycling of the lines. Such maintenance costs mayinclude repairing or replacing lines that become damaged as a result ofcyclic, thermally-induced expansion and contraction.

Still another feature of at least some of the foregoing embodiments isthat a heater can be co-located with a corresponding solar collector toreduce the length of the connecting line between the solar collector andthe overall steam outlet. As discussed above, this arrangement canreduce blow down, thermal losses, and/or the maintenance costs. Inparticular embodiments, a single heater may be coupled to a single solarcollector. In other embodiments, a single heater may serve multiplesolar collectors (e.g., arranged circumferentially around a commonheater). In still further embodiments, multiple heaters can service asingle solar collector.

FIGS. 8A-8C illustrate still further aspects of the disclosedtechnology. FIGS. 8A illustrates an injection system that uses a 2″ flowline and fixed chokes at the well heads to control steam flow. The fixedchokes rely on choked (sonic) flow to limit the flow through each pipeand provide a uniform flow over many pipes. Injection rates to the wellscan vary with steam generator pressure, with injection rates typicallylimited to about 1000 bbl/day. FIG. 8B illustrates a system withautomated (adjustable) chokes, and with the chokes moved from the wellsto the upstream manifolds. The flow lines are increased from 2″ to 3″ indiameter, producing injection rates up to about 2000 bbl/day. FIG. 8C,which will be discussed in greater detail later, illustrates oversizedflow lines configured to enhance the use of solar generated steam.

Typical oil field steam injection techniques include “cyclicstimulation” or “huff-and-puff” injection, in which steam is injectedinto all oil wells for some period of time. Each well is optionally leftfor an additional period of time to “soak”, and then oil and water areproduced back from the well, until production falls to a predeterminedrate, or a predetermined period of time has passed. The cycle—inject,soak, produce—is then repeated.

Another common approach involves continuous injection of steam in oneset of wells (“injectors”), and continuous production in adjacent wells(“producers”). Based on the spatial relationship of injectors andproducers such projects are known as steam floods, steam drives, SAGD(steam assisted gravity drainage), and by other names as well.

It is common in a particular oilfield to begin the overall steaminjection project with a period of cyclic injection in all wells, so asto produce early oil and also so as to warm the formation, improving thepermeability and communication between injectors and producers. Then itis common to begin steamflood continuous or semi-continuous injection inthe injection wells for an extended period of time. During the earlyperiod of the project, continuing injection of steam raises thetemperature of the formation surrounding the injection well. The highertemperatures and the flow within the formation away from the injectorwell of fluids, including oil, increases the “injectivity” of thewell—that is, increases the rate of steam flow into the injector wellfor a given wellhead steam pressure. As injectivity rises, in manysteamflood operations, to maintain a roughly constant daily mass ofsteam per injector, the surface steam pressure may be reduced.

A typical steamflood has a limited number of steam sources, e.g. one ora plurality of boilers—and a much larger number of injection wells. Animportant element to the success of any steamflood is the properdistribution of steam flow into the injection wells. Without properdesign for the distribution of steam, more steam might flow into thoseinjection wells which are located closer to the boilers (withcorrespondingly lower flow-related hydraulic pressure drop), and lesssteam would flow into wells which are relatively farther from theboilers. Such imbalance of steam distribution is undesirable, as itwould provide imbalance of formation heating and reduce both the rate ofproduction and the total recovery of oil. Accordingly, flow controldevices installed at each wellhead, or installed at each group of wells,are an important part of a steamflood operation. As noted above withreference to FIGS. 8A and 8B, fixed chokes are used in some contexts(steamflood) and automated adjustable chokes in others (cyclic).

As steam injection proceeds over months and years, the formationeventually heats and reaches an equilibrium or plateau condition, wherea steam space and heated region has been established. The rate of steaminjection required to maintain this heated zone is typically lower thanthe rate of steam injection which was required to heat the formationoriginally and overcome thermal losses. As a result, the total amount ofsteam per injector per day into a given injector well typically isreduced after this equilibrium condition has been achieved.

The operating characteristics of solar steam generators, whose outputvaries widely from zero flow overnight to peak flow at noon, are quitedifferent from the operating characteristics of fuel-fired steamgenerators, which typically are operated at a roughly constant rate ofsteam production for extended periods (weeks or months of continuousconstant-output operation). The interconnection of solar steamgenerators poses particular issues for the design of oilfield steamdistribution networks and steam injection wells, particularly as thedesired fraction of solar steam increases.

As discussed above, the output of a solar steam generator (e.g., a solarcollector) and a fuel-fired steam generator (e.g., a heater) can bebalanced—as solar steam output increases, the firing rate and steamgeneration rate of fuel-fired steam generation is decreased, so as toprovide an approximately constant total rate of steam production perhour. In such a roughly constant-rate steam production, the totalfraction of annual steam which can be provided by solar is limited bythe fraction of annual hours when suitable solar radiation is available.Typical figures are in the range of 20% to 30% of total annual steam.

It is of significant value to oilfield operators to derive a greaterfraction of their total annual steam from solar energy, without reducingthe rate of oil production from the field. This is accomplished inaccordance with embodiments of the presently disclosed technology byinjecting more steam during the day, and less at night. In particularembodiments, this includes allowing the peak steam flow rate to riseabove the daily average rate, and allowing the overnight rate of steaminjection to drop below the daily average rate.

This matter of daily rate variation of steam injection poses challengesfor both the design of the injection wells and the design of the steamdistribution network. The present disclosure addresses both topics.

Solar Fraction Versus Life of Field

Typically in a steamflood operation, the rate of steam injection perwell is established by the steam flow needed during the earliest periodof the steamflood—that is, the time when the formation is coldest, hasthe lowest injectivity (lowest flow versus pressure), and requires thegreatest amount of steam to achieve formation warming. A field isdesigned with a chosen number of injection wells, designed to handle aparticular steam pressure and flow, so as to economically improve (e.g.,optimize) the cost of steam injection versus the oil productionresponse. More injection wells on closer spacing, or higher steampressures and well bores, can of course result in higher rates of steaminjection; but such changes also result in higher cost. Those skilled inthe art commonly optimize the cost of facilities against the projectedvalue of the oil production expected from such facilities design, so asto arrive at a balanced design which trades cost against expectedrevenue.

If high-fraction solar steam were desired at the outset of thesteamflood, the high rate variation associated with solar steamproduction would face limited injectivity and high demand for steam perwell. Delivering a large amount of steam per hour against high injectionpressures would increase the cost of each injection well (larger bore,higher pressure) and/or the cost of the steam distribution network(higher peak pressure) and/or the number of injection wells (higher costdue to more injectors).

Accordingly, aspects of the present technology address this issue. Inparticular embodiments, solar steam is integrated into the oilfield atan increasing fraction as the steamflood progresses. The startingconfiguration of the steamflood design is optimized as it would be forconstant-rate steam injection. Fuel-fired steam generators are turneddown daily to roughly match the output of solar steam generators, sothat total steam production is roughly constant. As the formation heatsand injectivity rises, more solar steam generators are added to theoilfield, or fuel-fired steam generators are removed and/or operatedless. Without increasing the peak pressure in the steam distributionnetwork, the rising injectivity allows a higher peak steam flow and thusallows solar energy to provide a higher fraction of daily total requiredsteam. Put another way, higher injectivity allows greater flow rates atthe same or lower pressures. Solar energy can produce the same dailyflow rate as a fuel fired steam generator, but with a high flow rateduring the day, and a low or zero flow rate at night. Accordingly, asthe peak flow rate capacity of the system increases with increasedinjectivity, solar energy can provide a greater and greater fraction ofthe total required steam—it does so by producing more steam during theday and less (or none) at night, and accordingly is suitable in caseswhere the total flow per day is more important than a consistent flowthroughout the day.

As the requirement for mass of steam per well per day drops over monthsand years of steam injection, the solar steam fraction is increasedfurther, by installing more solar steam generation capacity and/orremoving or reducing the operation of fuel-fired steam generators. Thusan installation which might in its first year deliver only 20% solarsteam, by its fifth year of operation might be running 80% annual solarsteam fraction, without an increase of the number of wells, or anincrease in the pressure rating of the steam distribution network. On alifetime basis, then, this innovation optimizes (or at least improves)the total fuel saving (a key goal of solar steam generation) at a lowtotal capital cost in the steam distribution and injection network.

Variable-Rate Steam Distribution

Typical practice in the oilfield to accomplish the proper distributionof steam across multiple injection wells is the use of “chokes”, deviceswhich incorporate nonlinear pressure-drop phenomena at a chosen flowrate or pressure. Flowrate-limiting “fixed chokes” (FIG. 8A) deliver aroughly constant flow rate into a well, limiting the flow rate by flowphenomena associated with the speed of sound. Using fixed chokes allowsa steam injector close to the boiler, at a relatively higher wellheadsteam pressure, to deliver the same mass of steam per day down the wellas another similarly equipped injector at a relatively lower wellheadsteam pressure located farther from the boilers.

Such fixed chokes, however, are effective only for a limited range ofoperating pressures, and do not allow for the proper distribution ofsteam at varying flow rates. As an oilfield changes how it is operated,from total fixed-rate steam, and low solar steam fraction, to highlyvarying steam flow rate, and high solar fraction, fundamentally newmechanisms must be employed to assure proper distribution of steamacross multiple injection wells from a common source.

Motor-operated valves and/or motor-operated chokes (FIG. 8B) are incommon use for cyclic steam injection operations in the oilfield. Suchmotor-operated valves allow steam to be delivered from a commondistribution header into each well on a proper schedule as describedabove.

The present technology employs motor-operated valves to accomplish theproper distribution of steam injection across multiple wells at varyingflowrates. In particular embodiments, a controller is associated witheach valve, and a sensor is associated with each injector. A controlmechanism determines the appropriate rate of steam injection for a givenwell, senses the current flow rate of steam into the well, and adjusts amotor-operated valve or motor-operated choke so as to bring the rate tothe desired rate. The desired rate of steam injection into each well maybe calculated locally by a controller based on a fixed schedule (hour ofthe day), based on a signal transmitted from a central controller, orbased on measurements of characteristics of the steam distributionnetwork. In some embodiments all steam injection rates are commandedfrom a central controller based on the current availability of solarsteam and fuel-fired steam, and information may be broadcast to wellheadcontrollers. In other embodiments, a communications path may exist froma central controller to each wellhead controller and allow fineradjustment of flowrate per well. In other embodiments, local wellheadcontrollers may sense characteristics of the steam distributionnetwork—e.g. variations in pressure—and may cause the controller to moveto a defined higher flowrate, than the flowrate at a relatively lowerpressure in the steam distribution network. Each wellhead controller maysense the mass flow of steam into its associated well by one of severalmeans, including coriolis-type mass flow meters, sensors which measurethe relative pressure on each side of an orifice plate, sensors whichemploy rotating elements such as turbines to sense flow velocity, orsensors which employ other characteristics such as magnetic flow meters,to sense the mass flow currently entering each injection well.

FIG. 8C illustrates a system design which anticipates and exploits thechange in field injectivity using selective pipe sizing and automatedchokes, in accordance with an embodiment of the present technology. Inone aspect of this embodiment, the flow lines are oversized compared tothe initial steam flow rate through the system. In other words, theinitial low injectivity of the rock formation, rather than the pipesize, limits the flow rate. As the injectivity increases, the largerpipes can accordingly handle an increased flow rate. As discussed above,such an increased flow rate can allow solar generators to produce asufficient daily flow rate of steam by delivering a large quantity ofsteam during daylight hours and no or low quantities of steam at night.The automated valves can be used to individually control the flow rateto individual wells so as to improve the uniformity of the steamdelivery process, even in a steamflood (as opposed to cyclic) operation.The automated valves can also be employed to support any of a variety ofother suitable variations.

From the foregoing, it will be appreciated that specific embodiments ofthe present technology have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the present technology. For example, while the heater 120is shown as including a single burner 111, in other embodiments, theheater 110 can include multiple burners. For example, a multi-burnerconfiguration can include a larger burner and a smaller burner so as toaccommodate deeper turndown. A potentially significant feature of amulti-burner configuration is the ability to maintain of a particularprofile of NO_(x) generation during turndown operation. The first andsecond heater portions 113, 114 are illustrated as single pass, foldedtubes. In other embodiments, either or both of these heater portions caninclude any of a variety of parallel, serial, and/or other suitable heatexchanger arrangements. For example, the first portion 113 and/or thesecond portion 114 can, in at least some embodiments, have two or moreparallel sections, each of which can separately deliver heated water tothe solar collector 120. This arrangement can reduce the overallpressure drop of water passing through the heater 110 and will typicallyinclude appropriate valving and/or other management features to properlybalance the flows. In still further embodiments, the heater 110 caninclude more than two individually controllable portions. In any ofthese embodiments, the heat exchange surfaces of the heater portions areappropriately sized to enhance efficiency at low cost. The heater 110can be a direct-fired boiler, a heat recovery steam generator (e.g.,capturing waste from a fuel-fired turbine, and/or a heat recovery steamgenerator with auxiliary fuel firing (e.g., a “duct burner” or“supplemental firing” arrangement). Aspects of the technology disclosedherein can be applied to “once through” steam generators, and/or torecirculating steam generators, depending on the embodiment. Otherarrangements for heaters, solar collectors, and associated componentsare disclosed in the following published PCT applications, assigned tothe assignee of the present application, and incorporated herein byreference: WO2010/088632; WO2012/006255; WO2012/006257; WO2012/006258;and WO2012/128877.

Certain features have not been shown or described in detail herein forpurposes of clarity. Such features include but are not limited toadditional isolation valves at the solar collector 120 (to allow thesolar collector to be drained for maintenance), and/or other suitablemaintenance and non-maintenance features.

Certain aspect of the technology described in the context of theparticular embodiments may be combined or eliminated in otherembodiments. For example, particular embodiments can operate in fewerthan all of the modes described above, and/or can include differentvalve arrangements. The exhaust gas recirculation feature can beeliminated in some embodiments. Features discussed in the context of anyof the Figures (e.g., FIGS. 7A-7E) may be combined in a suitable mannerwith features disclosed in other Figures (e.g., FIGS. 2A-2E). Particularaspects of the technology are embodied in steam production systems andassociated methods for producing steam. Other embodiments includemethods of making, configuring, adjusting and/or changing such systems,as well as methods for instructing these techniques. Further, whileadvantages associated with certain embodiments of the technology havebeen described in the context of those embodiments, other embodimentsmay also exhibit such advantages, and not all embodiments neednecessarily exhibit such advantages to fall within the scope of thepresent technology. Accordingly, the present disclosure and associatedtechnology can encompass other embodiments not expressly shown ordescribed herein.

1-41. (canceled)
 42. A enhanced oil recovery system, comprising: acontroller having a computer-readable medium programmed withinstructions that, when executed: direct steam into an injection well toproduce a first daily steam injection rate when the injection well has afirst injectivity, with a first fraction of the steam produced by solarheating with concentrated solar energy; and direct steam into theinjection well to produce a second daily steam injection rate when theinjection well has a second injectivity greater than the first, with asecond fraction of the steam produced by solar heating with concentratedsolar energy, the second fraction being greater than the first fraction.43. The controller of claim 42 wherein the first fraction is a firstfraction of an annual steam production for a first year, and wherein thesecond fraction is a second fraction of an annual steam production for asecond, subsequent year
 44. The controller of claim 42 wherein thesecond daily steam injection rate is approximately the same as the firstdaily steam injection rate.
 45. The controller of claim 42 wherein thesecond daily steam injection rate is lower than the first daily steaminjection rate.
 46. The controller of claim 42 wherein directing steaminto the injection well to produce the first daily steam injection rateis performed for a first number of hours per day, and wherein directingsteam into the injection well to produce the second daily steaminjection rate is performed for a second number of hours per day, thesecond number of hours per day being less than the first number of hoursper day.
 47. The system of claim 42 wherein the controller isoperatively coupled to one or more valves that control the first andsecond daily steam injection rates.
 48. The system of claim 42 whereinthe controller is operatively coupled to one or more valves to changefrom the first daily steam injection rate to the second daily steaminjection rate as a function of injectivity.
 49. The system of claim 42wherein the controller: directs steam into the injection well at anapproximately constant flow rate of steam per hour when the injectionwell has the first injectivity; and directs steam into the injectionwell at a variable flow rate of steam per hour when the injection wellhas the second injectivity.
 50. The system of claim 49 wherein thevariable flow rate of steam per hour includes a first rate during theday and a second, lower rate at night.
 51. The system of claim 49wherein the variable flow rate of steam per hour includes a positiverate during the day and a zero rate at night.
 52. The system of claim 42wherein the controller directs steam from both a solar collector and afuel-fired heater.
 53. The system of claim 52, further comprising: thesolar collector, wherein the solar collector includes a plurality ofsolar concentrators; and the fuel-fired heater.
 54. The system of claim42 wherein the controller directs steam from a greater number of solarcollectors when the injection well has the second injectivity than whenthe injection well has the first injectivity.
 55. The system of claim 42wherein the controller directs steam from both a solar collector and afuel-fired heater when the injection well has the first injectivity, andwherein the controller directs less heat from the fuel-fired heater whenthe injection well has the second injectivity than when the injectionwell has the first injectivity.
 56. The system of claim 42 wherein thecontroller directs steam from both a solar collector and a fuel-firedheater when the injection well has the first injectivity, and from thesolar collector but not the fuel-fired heater when the injection wellhas the second injectivity.
 57. The system of claim 42 wherein thecomputer-readable medium is programmed with instructions that, whenexecuted, direct steam into the injection well at a first pressure whenthe injection well has the first injectivity, and direct steam into theinjection well at a second pressure when the injection well has thesecond injectivity, the second pressure being approximately the same asthe first pressure.
 58. The system of claim 42 wherein thecomputer-readable medium is programmed with instructions that, whenexecuted, direct steam into the injection well at a first pressure whenthe injection well has the first injectivity, and direct steam into theinjection well at a second pressure when the injection well has thesecond injectivity, the second pressure being less than the firstpressure.
 59. The system of claim 42, further comprising a network ofpipes coupled to the injection well to deliver steam to the well, andwherein the pipes have excess capacity when the injection well has thefirst injectivity.
 60. A enhanced oil recovery system, comprising: asolar collector that includes at least one solar receiver coupled to anoilfield injection well, and at least once solar concentrator positionedto direct solar radiation to the at least one solar receiver; afuel-fired heater coupled via a conduit to the oilfield injection well;and a controller having a computer-readable medium programmed withinstructions that, when executed: direct steam into the injection wellto produce a first daily steam injection rate when the injection wellhas a first injectivity, with a first fraction of the steam produced bythe solar collector; and direct steam into the injection well to producea second daily steam injection rate when the injection well has a secondinjectivity greater than the first, with a second fraction of the steamproduced by the solar collector, the second fraction being greater thanthe first fraction
 61. The controller of claim 60 wherein the firstfraction is a first fraction of an annual steam production directed tothe oilfield injection well during a first year, and wherein the secondfraction is a second fraction of an annual steam production directed tothe oilfield injection well for a second, subsequent year.
 62. Thesystem of claim 60 wherein the controller directs steam from both thesolar collector and the fuel-fired heater when the injection well hasthe first injectivity, and wherein the controller directs less heat fromthe fuel-fired heater when the injection well has the second injectivitythan when the injection well has the first injectivity.
 63. The systemof claim 60 wherein the controller directs steam from both the solarcollector and the fuel-fired heater when the injection well has thefirst injectivity, and from the solar collector but not the fuel-firedheater when the injection well has the second injectivity.
 64. A methodfor oil recovery, comprising: instructing injection of steam into anoilfield injection well to produce a first daily steam injection ratewhen the injection well has a first injectivity, with a first fractionof the steam produced by solar heating with concentrated solar energy;and instructing injection of steam into the oilfield injection well toproduce a second daily steam injection rate when the injection well hasa second injectivity greater than the first, with a second fraction ofthe steam produced by solar heating with concentrated solar energy, thesecond fraction being greater than the first fraction.
 65. The method ofclaim 64 wherein steam is directed into the injection well to producethe first daily steam injection rate for a first number of hours perday, and wherein steam is directed into the injection well to producethe second daily steam injection rate for a second number of hours perday, the second number of hours per day being less than the first numberof hours per day.
 66. The method of claim 65 wherein: instructinginjection of steam into the injection well for the first number of hoursper day includes instructing injection at an approximately constant flowof steam per hour when the injection well has the first injectivity; andinstructing injection of steam into the injection well for a secondnumber of hours per day includes instructing injection at a variableflow of steam per hour when the injection well has the secondinjectivity.
 67. The system of claim 66 wherein the variable flow ofsteam per hour includes a first rate during the day and a second, lowerrate at night.
 68. The method of claim 66 wherein the variable flow ofsteam per hour includes a positive rate during the day and a zero rateat night.
 69. The method of claim 64 wherein instructing injection ofsteam when the injection well has the first injectivity includesinstructing injection from both a solar collector and a fuel-firedheater.
 70. The method of claim 64 wherein instructing injection ofsteam into the injection well for when the injection well has the secondinjectivity includes instructing injection from a greater number ofsolar collectors when the injection well has the second injectivity thanwhen the injection well has the first injectivity.